The present invention relates to a method for forming a fine through hole conduction portion of a circuit board by which a fine through hole can be assuredly formed to a flexible circuit board and the like requiring the high-density wiring.
As means for mutually conducting wiring patterns on both sides of a circuit board, it is common to form a through hole at a required position by a drill and the like and then perform through hole plating on the inner surface of the through hole.
Here, in order to form a wiring pattern with a higher density, it is advantageous to form a diameter of the through hole as small as possible. However, in case of the perforating process using the drill means, formation of a hole having a diameter of approximately 0.4 mm is the limit, which will be an obstacle when the high-density wiring is desired.
Further, in case of a multilayer circuit board, conduction between an internal layer wiring pattern and an external layer wiring pattern is also generally achieved by the through hole coating, but electrical connection of the internal layer wiring pattern is intended only by plating on the internal wall. With this structure, a contact area between the through hole coating and the internal layer wiring pattern is not very large, and a crack caused due to thermal expansion is hence apt to be generated. Therefore, it is hard to say that the reliability of connection conduction is excellent. Further, in the multilayer circuit board, as a technique for mutually conducting the external layer wiring pattern and the internal layer wiring pattern, there is disclosed a technique for forming a through hole by laser irradiating means through a mask to which a through hole is provided in, for example, Japanese Patent Application Laid-Open No. 78171/1974. The hole connected for conduction by such a technique is known as an SVH, i.e., a blind through hole or simply referred to as a via hole. Since this technique uses chemical resin etching means or laser means, fine hole processing is possible, but a through hole which pierces the circuit board is not obtained.
It is, therefore, an object of the present invention is to provide a method for forming a fine through hole of a circuit board, which can also preferably cope with a fine wiring pattern while preferably improving the connection reliability for conduction by quickly minifying mutual conduction of an internal layer wiring pattern and an external layer wiring pattern in a stepped structure in case of a multilayer circuit board.
To achieve this aim, a method for forming a fine through hole conduction portion of a circuit board according to the present invention is characterized in that, in order to realize through hole conduction between an internal layer wiring pattern and each external layer conduction layer on both sides, a through hole which is partially blocked is formed to said internal layer wiring pattern from one external layer conduction layer by laser means; another through hole which is also partially blocked and displaced from said through hole in an allowable range is formed to said internal layer wiring pattern from the other external layer conduction layer by said laser means to form a stepped through hole; and thereafter through hole plating is carried out.
Here, a pattern shape at a position where the through hole conduction is to be made in the internal layer wiring pattern is formed to be larger than a shape of a conduction layer removing hole formed to the both external layer conduction layers. Further, in the internal layer wiring pattern of the through hole conduction portion, a conduction layer such as a copper foil should be removed to be half the size of the shape of the internal layer wiring pattern. Moreover, both the regular electroless plating means and the electrolytic plating means can be used for the through hole plating processing.
According to such a through hole structure having the overlapping stepped form, since the contact area of the internal layer wiring pattern and the through hole coating can be preferably enlarged, the reliability of the electrical connection between the internal wiring pattern and the both external layer conduction layers can be greatly increased even with the fine through hole conduction structure, thereby obtaining the circuit board for the multilayer high-density wiring.